The PUTROW function writes the given array of bytes into the given lane
number and row number. The comments about numbering and data type
mentioned in GETROW also apply here. Note that in a newly created
bigimage, it is invalid to (say) PUTROW lane 1 row 1, then PUTROW lane 1
row 3. That is, one can only either overwrite existing rows, or append
the next row to the end of the lane. It is perfectly valid to write (say)
lane 5 without lanes 1 to 4 existing.